A meltblown fiber and fabric is typically made in a one-step process in which high-velocity air blows a molten thermoplastic resin from an extruder die tip onto a conveyor or takeup screen to form a fine fibered self-bonding web. Although many types of polymers can be used meltblown fibers and fabrics, polypropylene is one of the most commonly used polymers. In one meltblown process, the polypropylene is fed as granules or pellets into an extruder where it can be mixed with additives in a masterbatch. Additives include stabilizers, peroxides, dyes, and other chemical agents.
In one type of meltblown process, metering pumps are used to pump the molten polypropylene to a distribution system having a series of die tips, the polypropylene being in the molten state at some processing temperature. The die tip is designed in such a way that the holes are in a straight line with high-velocity air impinging from each side. A typical die will have 0.38 mm diameter holes spaced at 10 to 16 per cm (25–40 per inch). The impinging high-velocity hot air attenuates the filaments and forms the desired fibers, or microfibers. Immediately below or adjacent to the die, a large amount of ambient air is drawn into the hot air stream containing the microfibers which cools the hot gas and solidifies the microfibers onto a forming belt or other solid surface that is typically moving in such a manner as to create a continually renewed surface for the microfibers to contact and form a fabric or web (hereinafter “fabric”). The processing temperature is one factor in the final fabric properties. The “optimal” processing temperature is one at which ideal properties of the fabric are achieved such as low shot with good hand and high barrier properties, or good filtration properties.
Fabric quality is controlled by many factors such as the uniformity of the fiber distribution in the air stream and adjustment of other manufacturing variables such as processing temperature. Another factor controlling fabric quality are the properties of the polypropylene. Several defects can result from a poor selection of polymer physical properties such as roping, fly, and shot. Shot are pinpoint-sized polypropylene beads in the fabric that affect porosity, uniformity, and hand (texture) of the fabric, and are a principle cause of off-quality fabric. More particularly, too broad a molecular weight distribution, and/or slow crystallization of the polymer can cause shot.
The presence of shot decreases the hydrohead of the fabric, which is a measure of the porosity (liquid barrier resistance) of the fabric. This is undesirable in many of the desired applications of polypropylene fabrics such as surgical barriers and protective wear for medical personnel, where there is a need to prevent fluid-borne pathogens from directly contacting the medical personnel, and is undesirable in filtration devices as well. Polypropylene produced by Ziegler-Natta catalyzed processes have been used to produce fibers and fabric, but have the drawback of requiring the use of peroxides as a pre-treatment to forming the fibers and fabric. The addition of peroxides increases the complexity and expense of making the fibers, such as by adding more variability to the process of manufacturing the fibers and fabrics, and is hence undesirable.
Polypropylene produced by metallocene catalyzed processes have also been used to make fibers and fabric, such as disclosed by Stahl et al. in U.S. Pat. Nos. 5,723,217; 5,726,103; 5,736,465; 5,763,080; and 6,010,588, which describe a benzo-substituted indenyl metallocene using an alkylalumoxanes as an activator. Richeson et al. in U.S. Pat. No. 5,891,814, disclose a dual metallocene-generated propylene used to make spunbond fibers, also using an alkylalumoxanes activator. Also, WO 99/19547 discloses a method for producing spunbonded fibers and fabric derived from a blend of propylene homopolymer and a copolymer of polypropylene. While these metallocene catalyzed polypropylenes are reactor grade (not treated with peroxides), shot is still at an unacceptable level in these polypropylenes.
What is needed is a meltblown fabric that has low shot levels, and maintains a high hydrohead and soft hand. Also, fabrics with low shot levels useful for articles such as filtration devices and barriers.